This study will address fundamental aspects of participation in Medicare Part D that will inform policy related to insurance access and impacts. Now that several years of data are available on Part D participation, a more in- depth look at the effectiveness of the program in providing access to drugs for eligible participants is possible. A thorough examination of insurance choice in the context of Part D requires a simultaneous understanding of participation in its premium and cost sharing support program, the Low Income Subsidy (LIS). The LIS provides additional subsidies for Part D enrollees and represents an effort to expand access to prescription drugs for those who might otherwise be unable to afford them. A key characteristic of both Part D and the LIS is that enrollment is voluntary. Consequently, to examine the efficiency of the program we must understand enrollment decisions. This study will focus on three fundamental issues surrounding the efficiency of Part D as a public health policy. Specifically, I will assess the effectiveness of the program in (1) enrollment of beneficiaries eligible to receive the LIS, (2) the impact on drug utilization among LIS participants, and (3) the subsequent enrollment among beneficiaries who failed to enroll at their first opportunity. The first two aims of this study will use the Medicare Current Beneficiary Survey (MCBS). Low enrollment in the LIS is a major policy concern;however, estimates of eligibility and take-up are unreliable, in part due to the lack of a single data source that captures both dimensions. The objective of Aim 1 is to develop better estimates of whom, among likely eligible beneficiaries, enrolled in Part D with LIS or not and whether non-enrollees have identifiable characteristics that would improve the ability to reach them. An unpublished Income and Assets Supplement to the MCBS will be used in conjunction with an income imputation procedure to enhance the financial measures used to determine eligibility. In addition, the impact of LIS on drug utilization among non- Medicaid duals is unexplained and hindered by endogeneity in LIS participation. Building on Aim 1, the second aim will use the MCBS with enhanced eligibility measures to estimate the effect of LIS enrollment on drug utilization. A regression discontinuity design will be used to address potential bias resulting from self-selection. The final aim will assess the efficiency of the Part D design in providing an incentive for timely enrollment and a disincentive to not wait until the perceived need to enroll is evident. In the enrollment decision, there is a trade- off between the savings in premiums not expended by non-enrollees and subsequent financial penalties from late enrollment. This aim will examine whether that trade-off is sufficient to induce timely participation among non-duals by examining the role of health shocks on late enrollment using 2006-2008 data from the Center for Medicare and Medicaid Services Chronic Condition Warehouse. Study findings will provide a comprehensive evaluation of critical components related to participation in Medicare Part D and inform policy on three topics unaddressed in current literature. PUBLIC HEALTH RELEVANCE: This project assesses the role of public policy in access to and impacts of prescription drug insurance in the context of Medicare Part D. It takes a unique look at key features of the program design to examine classic insurance participation issues. Specifically, this study will explore take-up of the Low Income Subsidy (LIS), the impact of LIS enrollment on drug utilization, and late enrollment into Part D.